1. Field of the Invention
The present invention relates to a method of controlling print registration in an electrophotographic machine, and, more particularly, to a method of controlling laser scanner phase in a multicolor electrophotographic machine.
2. Description of the Related Art
In an in-line color laser image printing process, the print medium typically passes through four color developing stations in series, with the colors being black, magenta, cyan and yellow. In order for the multi-color laser printer to print at the same speed as a monochrome laser printer, photoconductive drum exposures must occur for all four colors simultaneously. The color planes must be properly registered with each other for proper rendering of images without undesirable color shifts and artifacts.
Pipeline color printers which image using laser print heads are limited by the granular one picture element separation between successive scan lines. Without a method of controlling the phase relationship between multiple laser scanners, color planes would be randomly shifted relative to each other by a distance between zero and the width of a picture element, thereby creating color shifts. Thus, alignment of the four color developing stations in both the process direction (feed direction of the print medium) and scan direction (across the page) is critical.
The process location of each scanning laser beam must overlap in order to prevent color offset in the process direction. Each color must have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. Without such adjustments, after a scan line is transferred from a first photoconductive drum to the transfer belt, and that point on the transfer belt is rotated to a second photoconductive drum, the scan line on the transfer belt from the first photoconductive drum will not align in the process direction with a scan line to be transferred from the second photoconductive drum.
What is needed in the art is a method of aligning scan lines of multiple photoconductive drums in a process direction in order to avoid undesirable color shifts and print artifacts.